ICP-OES, MIP-OES, ICP-MS and MIP-MS are well known techniques for elemental analysis of samples providing quantified determinations of elements present in liquid or solid samples trace levels (ppb or ppt). Small particles of solid sample or droplets of liquid sample are introduced into a plasma whereupon they are atomized, excited, and a proportion are ionized. Excited atoms and ions emit photons characteristic of the elements present and the optical emission produced is analyzed in ICP-OES and MIP-OES using an optical spectrometer. Ions are introduced into a vacuum system containing a mass spectrometer for mass analysis.
In the case of inductively coupled plasmas, the plasma is produced in an inert gas, usually argon, by means of the inductive coupling of power from an RF current driven through a coil surrounding the inert gas. The inert gas is present in the form of a flow of gas through a torch, the coil surrounding the torch at a gas outlet from the torch. Hence the torch has an axis along which the gas flows, and the coil has an axis; usually the torch and the coil are arranged coaxially. The torch usually comprises three concentric coaxial tubes, each of which is supplied with a continuous flow of inert gas. A continuous plasma may be formed, and sample is introduced into the plasma along the torch axis in the innermost concentric tube of the torch.
Microwave induced plasmas utilize GHz frequency radiation coupled using a waveguide to a cavity in connection with a torch. Continuous plasma powers of a few hundred Watts to over 1 kW may be generated. Various different types of physical arrangements of waveguides, cavities and torches have been used. In common with ICP apparatus, an inert gas, usually helium, nitrogen or argon, is typically used to form the plasma (although high levels of impurity can be tolerated so that air can be used), and sample material in the form of small droplets, small solid particles, or gases are passed into the plasma for excitation and ionization.
Where the sample is in liquid form, droplets of the liquid are produced using a nebulizer or a droplet dispenser, and the droplets are entrained in the flow of inert gas which supplies the innermost concentric tube of the torch. The droplets of liquid are taken into the axial region of the plasma by the gas flow, whereupon they are desolvated, atomized, ionized and excited.
Where the sample is in solid form, small particles of solid are liberated from the sample by, for example, laser or spark ablation, and carried into the axial region of the plasma by entrainment in the inert gas supplying the innermost concentric tube of the torch. The solid particles receive sufficient energy from the plasma to cause atomization, ionization and excitation.
The excited sample atoms relax to lower electronic states by the emission of photons, and the energy of the photons (the wavelength of the optical emission) is characteristic of the elements from which they came. In ICP-OES and MIP-OES, optical emission from the sample material in the plasma is directed, using optical elements such as lenses and mirrors, onto an optically dispersive element, such as a grating, and dispersed photons arrive at one or more detectors in the form of spectral lines, separated from one another in space. The use of spatially resolving detectors such as an array of CCD or CID detectors enables simultaneous detection of a spectrum or parts of a spectrum.
Optical emission from the plasma may be viewed along or near the axis of the plasma (axial viewing), or optical emission from the plasma which is emitted radially from the plasma may be viewed, (radial viewing). Some spectrometers have facilities for both axial and radial viewing. The spectrometer is usually controlled by one or more computers.
Quantification of the amount of an element present in the sample material is determined by measuring the intensity of one or more of the spectral lines related to that element, the number depending on how many elements are to be quantified and the degree of spectral and chemical interferences. For a single element analyte a simple calibration curve can be constructed relating the intensity to the concentration. When multiple elements are present there is the possibility of spectral interference (where one spectral line overlaps another). Typically this is corrected by measuring the interfering element at two lines, one of which is not interfered with. The level of interference at the line of interest is calculated by multiplying the intensity at the non-interfering line by a constant.
In ICP-MS and MIP-MS, a mixture of atoms, ions and plasma gas is admitted into successive stages of a vacuum system. In the first stage the plasma plays upon a cooled metal cone, the cone positioned upon the axis of the plasma. The cone has an orifice large enough to enable a boundary layer of the plasma to be penetrated so that material from the plasma passes into an evacuated expansion chamber. The material expands in the lower pressure region and forms a supersonic jet. A portion of the jet is skimmed using another metal cone into the next vacuum stage maintained at a lower pressure, where typically it encounters an ion extraction lens. Electrons are turned out of the beam path due to the electric field produced by the lens. Often the remaining beam of ions and neutral particles is then admitted to an ion-neutral separator which diverts charged particles through a path using electric fields, whilst neutral particles are unaffected. Ions are then admitted to a further vacuum chamber at still lower pressure, in which a mass analyser, such as a quadrupole mass filter, is located, together with a detector. Other, more complex systems including collision/reaction cells and/or different types of mass analyser are also well known. Mass analyzers separate the different ions on the basis of their mass-to-charge ratio. For simplicity, and as most ions are singly charged, this will herein be referred to as the mass of the ions.
When a continuous source of sample material is passed into the plasma, the stability of the detected optical signals from sample material (ICP-OES, MIP-OES) and the stability of the detected ion mass signals (ICP-MS, MIP-MS) are dependent, amongst other things, upon the stability of the plasma.
In most ICP-OES and ICP-MS instruments the plasma stability is controlled by controlling the RF power driven through the coil which generates the plasma. The RF power is supplied to the coil from an RF generator, the power being around 1 kW. At these power levels it is important to match the impedance of the RF generator to the impedance of the combination of any electrical conductors used together with the coil and the plasma. Changes in the plasma cause the impedance of the coil and plasma combination to change, and for efficient power transfer, the impedance of the RF generator must be altered.
Presently, widely used RF generators either use a crystal-controlled frequency drive together with a variable impedance matching network, or they use a free-running drive in which the frequency of the RF is modified to obtain a matching impedance. In the former case, the matching network is usually electromechanical and so it is slow to respond to changes in conditions. In the latter case, the free running generators automatically modify their frequency in response to changes in the load. The change in frequency causes a change in power delivered to the plasma, which is undesirable, but this may be compensated for by the instrument controller which may monitor the voltage and current driven through the coil.
It has been found that controlling the plasma power in this way does not however fully stabilize the detected optical and mass signals from sample material. Whilst the plasma power may be maintained constant to a high degree, the temperature of the plasma may change due to changes in the temperature or pressure of one or more of the gas flows fed into the torch, for example, or by the introduction of a different sample. Chemical and matrix interferences are a function of temperature therefore controlling the temperature rather than the power allows more accurate corrections to be applied.
An improved method of plasma control for an ICP-OES instrument was described in Japanese patent application JP06109639A. This method attempted to monitor the emission intensity of the plasma by monitoring the intensity of a single optical line and using a feedback system to control the RF generator power output so that the monitored line intensity remained constant. However the present inventors have found that RF power control to maintain intensity of a single line is sensitive to attenuation of light in the optical path between the plasma and the detector. The transmission of optical emission from the plasma varies over time due to contamination of optical elements, and this method of plasma control is unsuitable to provide significant benefits over other prior art methods involving RF power control.
Against this background the present invention has been made.